1. Field of the Art
This invention is intended to provide a process for producing a highly water absorptive polymer having excellent water absorbing properties and high water absorbing rate with extreme ease and at low cost.
The polymer obtained according to the production process of the present invention can be swelled by absorption of a large amount of water within a short time and is insoluble in water, and has a great strength of the swelled gel, and therefore it can be used advantageously for production of various absorbing materials and various materials which are to be used under swelled state with water being absorbed.
2. Prior Art
Materials such as paper, pulp, nonwoven fabric, spongy urethane resins and the like have hitherto been used as water retentive materials for a variety of sanitary goods such as a sanitary napkin, paper diaper and the like and a variety of agricultural materials. However, these materials have a water absorption capacity of no more than 10-50 times their own weight, which causes problems that an extensively increased bulk of the material is required for absorbing or retaining a large amount of water and that water is easily released from the material in which water has been absorbed on pressing it.
There have recently been proposed a variety of highly water absorptive polymer materials and production methods thereof in order to settle the aforementioned problems of the water absorptive materials of this kind. For instance, there have been proposed a graft polymer of starch (Japanese Patent Publication No. 46199/78, etc.), a denaturated cellulose (Unexamined Published Japanese Patent Application No. 80376/75, etc.), a crosslinked water soluble polymer (Japanese Patent Publication No. 23462/68, etc.), a self-crosslinking polymer of an alkali metal salt of acrylic acid (Japanese Patent Publication No. 30710/79, etc.), crosslinked type polyacrylic acid alkali metal salt (Unexamined Published Japanese Patent Publication No. 71909/1983, Japanese Patent Publication No. 17328/1985), and a process for producing a crosslinked polyacrylic acid salt by means of an aqueous redox polymerization (Unexamined Published Japanese Patent Publication Nos. 71907/1983 and 18712/1984).
However, some of these highly water absorptive polymer materials have a number of problems in practical use or production on an industrial scale such that they are still insufficient in amount of water to be absorbed, with the gel strength after water absorption being small, and that the polymer obtained by drying is extremely hard and cannot be easily crushed thereby requiring a great mechanical crushing force. Also, some of those mentioned above employ a large amount of hydrocarbon type solvent, thus involving a serious problem on safety.
Further, as far as the present inventors know, most of the polymers obtained by the production processes described in the specifications mentioned above cause the so called "gel blocking" phenomenon when contacted with water, whereby water absorption with good efficiency cannot be effected, thus having the drawback of requiring long time for water absorption. Therefore, most of the polymers require a post-treatment step of surface crosslinking with a crosslinking agent such as polyglycidyl ether.
Moreover, in most of the production processes as mentioned above, the polymerization time or the treatment time in the above mentioned post-treatment is long and, in addition, the process is complicated, whereby productivity is poor, thus inevitably leading to increased cost.